Liver regeneration following 70% partial hepatectomy leads to rapid activation of genes in the remnant liver. Interleukin-6 deficient (IL-6 −/−) mice have impaired liver regeneration and abnormalities in immediate early gene expression. In this study, the gene expression program in the IL-6 +/+ and −/− livers at 2 hours posthepatectomy was examined with a cDNA array representing 588 highly regulated mouse genes. Thirty-six percent of the 103 immediate early genes were induced differently in IL-6 +/+ compared with IL-6 −/− livers, implying regulation by IL-6. IL-6 treatment of the IL-6 −/− mice in the absence of hepatectomy induced a much smaller set of genes in the liver, suggesting that IL-6 cooperates with other hepatectomy-induced factors to activate the large number of genes. Northern blot analyses were used to verify gene expression data obtained from the arrays. The expression of urokinase type plasminogen activator receptor (uPAR) and plasminogen activator inhibitor-1 (PAI-1), critical components of the urokinase plasminogen activator (uPA) system, was lower and delayed in IL-6 −/− livers. Despite the fact that active uPAR/uPA complex is critical for hepatocyte growth factor (HGF) activation, no differences were detected between the IL-6 +/+ and −/− livers in HGF activation as measured by receptor phosphorylation. On the contrary, the mitogen-activated protein kinase (MAPK) pathway was activated in IL-6 +/+ livers early during regeneration but remarkably delayed in IL-6 −/− livers. Defective liver regeneration may be explained by the large number of gene activation pathways altered in IL-6 −/− livers and further supports the finding that IL-6 is necessary for normal liver regeneration.